Some time ago it was discovered that quadrature coils that is, polar coils, enable improving the sensitivity of the MR systems by a factor of at least the square root of two. With the usual linear coils, use is made of only half of the radiated energy, while quadrature coils enable using all of the radiated energy. Accordingly, the use of quadrature coils has increased.
For an example of quadrature coils, see the patent application filed in Israel on 20 February, 1986, which received Serial No. 77937 and was subsequently filed in the United States where it issued on July 12, 1988, as U.S. Pat. No. 4,757,290.
The invention of that Application is assigned to the assignee of the invention of this Application. The sensitivity improvement afforded by quadrature coils is described in an article entitled "Quadrature Detection Coils--A Further .sqroot.2 Improvement in Sensitivity", which appeared in the Journal of Magnetic Resonance, Volume 54, pp. 324-327 (1983). The article points out how the improvement in sensitivity occurs and provides two examples of quadrature combines for use in combining the signals received from each of the quadrature coils, i.e., coils that are orthogonal to each other. The quadrature combines phase shift signals or pulses received or transmitted by the quadrature coils to combine the quadrature signals and effect the improvement in sensitivity.
The prior art quadrature combines included a plurality of either active elements such as switches or diodes and/or a plurality of inductors and capacitors.
Using a multiplicity of active elements over which the signal to and from the quadrature coil flows of course decreases the reliability of the system. The plurality of elements, active and inactive used in the prior art also increases the cost of the prior art transmitting and receiving systems.